The easiest way to get the gun is to run the first dark below mission. Load it up and sprint to the blade of crota. Smash him and you shall receive. I was luck enough to get it on my 2nd try!
I also received my 2nd one from doing the blades of crota bounty on earth.
Thats twice I've received it from completing the bounty. Coincidence?
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#Destiny
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it's RNG so there is no simple way